The single-phase or multiphase generator constituting the conventional motor vehicle alternator generally has a stator within which rotates a rotor carrying an excitation coil. The coil is connected to brushes in contact with two slip rings on a projecting part of the rotor shaft.
EP-A-0 707 374 discloses rotating machines in which, for improved efficiency, the rotor excitation field is produced by permanent magnets and by coils (the expression "mixed excitation" is generally used), and in which the current delivered by the armature is controlled by excitation coil switching means which selectively reverse the excitation direction to reduce or even substantially eliminate the flux from the magnets.
This need to reverse the direction of the excitation current imposes the use of a so-called "H" semiconductor switching bridge, which is costly and adds to the unit cost of the machine.